Silica, dye yield new kind of quantum dot

Portland, Ore. - An alternative to quantum dots-encapsulating organic dyes in a silica matrix-has been developed by researchers at Cornell University. The process, they said, could cut the cost of making optical computing devices, and render them chemically inert as well.

The Cornell approach is a departure from the way most quantum dots are fabricated: nanoparticles being doped with heavy metals like cadmium-selenium.

"We have encapsulated multiple organic dyes in the core of a nanoparticle," said Ulrich Wiesner, professor of materials science and engineering. "The core is then encapsulated into a pure silica shell for protection." The core-shell architecture can be used in applications ranging from flat-panel displays to medical imaging to sensor and optical lasers that emit a single photon at a time, he said.

Wiesner's nanoparticles, which he calls "Cornell dots," are novel. They begin with a core 2.2 nanometers in diameter that contains a few colored dye molecules. The molecules are surrounded with 22.8 nm of silicon dioxide, resulting in quantum dots measuring 25 nm in diameter. This core-shell architecture, Wiesner said, makes his quantum dots as much as 30 times brighter than conventional fluorescent dyes.

"The particles are very, very bright, because they act independently rather than quenching each other," he said. "Our dots are almost as bright as quantum dots."

The silicon dioxide-or silica-shell also prevents the dyes from fading, while allowing a variety of colors to be produced without changing the diameter of the core. The silicon dioxide-coated nanoparticles are also chemically inert, making them safer to manufacture and handle. "We change the color of these particles to almost anything, by just changing the organic dyes that are inside their core," said Wiesner.

"Silica is benign, cheap and easy to coat onto nanoparticles," he said. "Our process is also compatible with silicon chip-manufacturing techniques, opening the door to applications in the life sciences and information technology."

Their brightness is probably explained by the dye molecules' being held stationary by the silica, eliminating lattice vibrations that would normally absorb photons. "We have chemically bonded the dyes to the silica, which makes them stiffer," Wiesner said. "That translates into higher quantum efficiency-which is the ratio of the incoming light to the outgoing light."

Wiesner is now building larger Cornell dots with a surface treatment that makes them mirrored, reflecting the emitted light back toward the core. He postulates that by using this method, he can create single-photon lasers, which would be ideal for quantum computing.

"We also work with particles in the micron regime," said Wiesner. "There, if you pipe light into the bead with a fiber, the light becomes trapped in the bead because it is reflected back whenever it gets to the surface. If you do it right, this can actually lead to lasing-potentially enabling single-particle lasers-essentially single photon sources."